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Dhairya Shah 15Dhairya Shah 15 

Which data type should of LastAcivityDate?

Challenge Not yet complete... here's what's wrong: 
The 'Days_Since_Last_Update__c' formula field did not return the correct number of days between an Account’s Last Activity Date and today.

i m having this error so i think i am using wrong data type for LastActivityDate so please help me out
sfdcMonkey.comsfdcMonkey.com
tha datatype of field is Formula 
your return type of field is Number .
it's looks like below image
User-added image
i hop it helps you
let me inform if it works
and mark it best answer if it helps you so it make proper solution for others :)